Sound localization cues are created by filtering properties of the head and pinna, which are known collectively as the head-related transfer function (HRTF). Because the HRTF is directionally dependent, unique patterns of energy peaks and troughs are added to the amplitude spectrum reaching a listener's eardrum as the source of a complex sound moves from one spatial location to another. The adaptive premium placed on accurate sound localization has exerted strong evolutionary pressure to develop auditory mechanisms for processing these spectral cues. Our longterm objective is to understand how such specializations influence auditory information processing within the central nervous system. Vocal communication systems, including human speech, convey meaning through similar patterns of spectral variation. In this context, proposed experiments will lead to a better understanding of how other biologiCally relevant sounds are first transformed by filtering properties of the ear and then processed by neural circuits within the brain. Expected results will build upon our extensive knowledge of localization behaviors in cats by examining how performance in sound localization tasks is influenced by manipulations such as blocking one ear, lesioning afferent or efferent pathways, and introducing background noise. Experiments of Aim l will test the hypothesis that binaural processing of spectral cues is necessary for accurate orientation toward sound stimuli. Experiments of Aim 2 predict that the dorsal cochlear nucleus performs a critical spectral analysis of the localization cues inherent in HRTF-filtered sounds. Experiments of Aims 3 and 4 will combine electrophysiological and behavioral techniques to test the hypothesis that the olivocochlear efferent system enhances the auditory nerve representation of spectral localization cues, particularly in the presence of environmental noise. Animal psychophysical experiments are the most direct means for testing hypotheses about the function of neural systems. Our research design is particularly powerful in this respect because it integrates well-characterized sound localization behaviors, recently developed neurophysiological models, and natural sound patterns to address fundamental issues of auditory processing.